Make sure that resource_label_event.epic_id exists

What does this MR do?

resource_label_event.epic_id was added as part of EE version of db/migrate/20180726172057_create_resource_label_events.rb, but if user used CE version in that time and then upgraded to EE, then this column was missing.

What are the relevant issue numbers?

Closes #7705 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?